Low current-carrying capacity time lag fuse

ABSTRACT

An electric current-limiting fuse includes a fusible element in the form of a wire and means arranged along the wire which perform the dual function of producing significant time lags when the fuse is carrying overload currents, and of producing highly effective series multibreaks when the fuse is subjected to major fault currents.

United States Patent [191 Knapp, Jr.

[ Sept. 24, 1974 LOW CURRENT-CARRYING CAPACITY TIME LAG FUSE [75] Inventor: Edward J. Knapp, Jr., Newbury.

Mass.

[73] Assignee: The Chase-Shawmut Company,

Newburyport, Mass.

[22] Filed: Jan. 21, 1974 [21] Appl. No: 435,253

[52] US. Cl 337/163, 337/159, 337/166, 337/296, 337/297 [51] llnt. Cl. HOlh 85/14 [58] Field of Search 337/163, 166, 256, 296, 337/295, 292, 297, 228, 229, 231, 232, 158,

[56] References Cited UNITED STATES PATENTS 6/1914 Cook 337/297 2,856,488 10/1958 Kozacka 337/231 2,877,321 3/1959 Jacobs Jr. 337/297 3,425,018 l/l969 Kozacka 337/158 Primary ExaminerR. N. Envall, Jr. Attorney, Agent, or Firm-Erwin Salzer 57] ABSTRACT An electric current-limiting fuse includes a fusible element in the form of a wire and means arranged along the wire which perform the dual function of producing significant time lags when the fuse is carrying overload currents, and of producing highly effective series multibreaks when the fuse is subjected to major fault currents.

4 Claims, 2 Drawing Figures PAH-1mm SEP24I974 //7// 77 i w V V LOW CURRENT-CARRYING CAPACITY TIME LAG FUSE BACKGROUND OF THE INVENTION This invention relates to current-limiting back-up fuses for protective devices having separable contacts which are caused to separate on occurrence of protracted overloads, e.g. back-up fuses for electric motor starters. This invention refers in particular to electric fuses of the above description which have a small current-carrying capacity, or current rating. Such fuses ought to comply with the following requirements.

a. As long as the fuse is carrying overload currents it should have a significant time lag so that all overload currents are interrupted by the protective device supposed to be backed-up rather than by the fuse.

b. When the fuse is subjected to major fault currents its fusible element ought to form highly effective series multibreaks causing a rapid decline of the fault current from the peak value of let-through current to zero and thus minimizing arcing i -t values.

c. In the interest of economy the same means should be used to achieve the above performance characteristic (a) and the above performance characteristic (b).

There are many prior art back-up fuses for protective devices having separable contacts which perform satisfactorily. The important problem of providing such back-up fuses for very small current ratings, and in particular of meeting the above requirements (a) to (c), does not seem to have found any satisfactory solution in the prior art. The present invention is a satisfactory solution to theproblem which has been outlined above. This has been verified by an extensive test program.

SUMMARY OF THE INVENTION Current-limiting cartridge fuses embodying the present invention include a tubular casing of electric insulating material and a pair of electroconductive terminal elements arranged at the end of said casing and closing said casing. There is a pulverulent arc-quenching filler inside the casing and also an elongated strip of electric insulating material submersed in said arc-quenching filler. A plurality of spaced metallic overlays is arranged on one side of and affixed to said strip. A fusible element in the form of a wire has ends conductively connected to said terminal elements and has a portion intermediate said ends thereof which is wound substantially helically around said strip in such a way that each turn of said fusible element engages one of said plurality of said spaced metallic overlays. Fuses embodying this invention further include a plurality of solder joints each on one of said plurality of metallic overlays and each thermally connecting one point of said fusible element to one of said pluralityof metallic overlays.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is in part a side elevation of a fuse embodying this invention and in part a section taken substantially along 1-] of FIG. 2; and

FIG. 2 is in part a side elevation of the fuse structure shown in FIG. 1 and in part a section thereof taken substantially along II-ll of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, numeral 1 has been applied to indicate a tubular casing or fuse tube of electric insulating material. A pair of electroconductive terminal elements in the form of caps or ferrules is arranged at the ends of casing 1 and closes the latter. Casing 1 is filled with a pulverulent arc-quenching filler 7, preferably quartz sand. The elongated strip 3 of electric insulating material is arranged inside of casing 1 and submersed in the pulverulent arc-quenching tiller 7. Strip 3 is preferably of an insulating material evolving arc-quenching gases when subjected to the action of electric arcs, e.g., a melamine-glass-cloth laminate which evolves nitrogen under the action of electric arcs. Strip 3 is provided on one side thereof with a plurality of spaced metallic overlays 4 which are affixed to strip 3 and the opposite side of strip 3 is bare. Strip 3 may be made of copper-clad insulating material and the gaps 5 between contiguous overlays 4 may be formed by local removal of the copper layer initially covering the entire strip 3. Ferrules or caps 2 are conductively interconnected by a fusible element in the form of a wire 8 which is preferably of silver. The portion of silver wire 8 intermediate its ends which are conductively connected to ferrules 2 is wound substantially helically or in zig-zag fashion around strip 3. Each turn of wire 8 engages one of copper overlays 4. In other words, the portions of the turns of wire 8 situated on the same side as copper overlays 4 engage copper overlays 4 and the portions of the turns of wire 8 situated on the opposite side of strip 3, i.e., the side thereof having no metal overlays are in physical engagement with the surface of strip 3. Reference numeral 6 has been applied to indicate a plurality of solder joints each on one of said plurality of metallic overlays and each thermally connecting one point of wire 8 to one of copper overlays 4. Thus each overlay 4 and each solder joint 6 jointly form a heat sink for wire 8 and these heat sinks or lag blocks are arranged in spaced relation in a direction longitudinally of wire 8.

When the fuse is carrying overload currents the F1 losses generated in wire 8 are absorbed by the heat sinks or lag blocks 4, 6 resulting in considerable time lags.

On occurrence of major fault currents wire 8 fuses virtually simultaneously on all its points situated between heat sinks or lag blocks 4,6. The formation of these series multibreaks results in the generation of a relatively high initial arc voltage which remains relatively stable on account of the evolution of gases from the points of strip 3 where the latter is engaged by wire 8 and where series arclets are kindled. This results in minimizing the letthrough 1' 1 values of the fuse, i.e., the sum of the melting and arcing i -r values.

I claim as my invention:

1. An electric current-limiting cartridge fuse of the time lag type including in combination a. a tubular casing of electric insulating material;

b. a pair of electroconductive terminal elements arranged at the ends of said casing and closing said casing;

c. a pulverulent arc-quenching filler inside said casing;

d. an elongated strip of electric insulating material arranged inside said casing and submersed in said arc-quenching filler;

e. a plurality of spaced metallic overlays arranged on one side of and affixed to said strip,

f. a fusible element in the form of a wire having ends conductively connected to said pair of terminal elements and having a portion intermediate said ends thereof wound substantially helically around said strip in such a way that each turn of said fusible element engages one of said plurality of spaced metallic overlays; and

g. a plurality of solder joints each on one of said plurality of metallic overlays and each thermally connecting one point of said fusible element to one of said plurality of metallic overlays.

2. A current-limiting fuse as specified in claim 1 wherein said strip of electric insulating material is of an insulating material evolving arc-quenching gases when subjected to the action of electric arcs, and wherein said fusible element is in the form of a wire of silver.

3. A current-limiting fuse as specified in claim 2 wherein said strip of electric insulating material is of a melamine-glass-cloth laminate.

4. An electric current-limiting cartridge fuse of the time-lag type including in combination a. a tubular casing of electric insulating material;

b. a pair of terminal caps closing said casing at the ends thereof;

c. a filler of quartz sand inside said casing; d. an elongated strip of an insulating material evolving gas under the action of electric arcs arranged inside said casing and submersed in said filler of quartz sand;

e. one side of said strip being provided with a plurality of metallic overlays spaced in a direction longitudinally of said strip and the other side of said strip being bare;

. a fusible wire of silver having end portions and a g. a plurality of solder joints each connecting one of said plurality of metallic overlays and said fusible wire of silver. 

1. An electric current-limiting cartridge fuse of the time lag type including in combination a. a tubular casing of electric insulating material; b. a pair of electroconductive terminal elements arranged at the ends of said casing and closing said casing; c. a pulverulent arc-quenching filler inside said casing; d. an elongated strip of electric insulating material arranged inside said casing and submersed in said arc-quenching filler; e. a plurality of spaced metallic overlays arranged on one side of and affixed to said strip; f. a fusible element in the form of a wire having ends conductively connected to said pair of terminal elements and having a portion intermediate said ends thereof wound substantially helically around said strip in such a way that each turn of said fusible element engages one of said plurality of spaced metallic overlays; and g. a plurality of solder joints each on one of said plurality of metallic overlays and each thermally connecting one point of said fusible element to one of said plurality of metallic overlays.
 2. A current-limiting fuse as specified in claim 1 wherein saId strip of electric insulating material is of an insulating material evolving arc-quenching gases when subjected to the action of electric arcs, and wherein said fusible element is in the form of a wire of silver.
 3. A current-limiting fuse as specified in claim 2 wherein said strip of electric insulating material is of a melamine-glass-cloth laminate.
 4. An electric current-limiting cartridge fuse of the time-lag type including in combination a. a tubular casing of electric insulating material; b. a pair of terminal caps closing said casing at the ends thereof; c. a filler of quartz sand inside said casing; d. an elongated strip of an insulating material evolving gas under the action of electric arcs arranged inside said casing and submersed in said filler of quartz sand; e. one side of said strip being provided with a plurality of metallic overlays spaced in a direction longitudinally of said strip and the other side of said strip being bare; f. a fusible wire of silver having end portions and a center portion, said end portions being conductively connected to said terminal caps and said center portion zig-zagging between said one side of said strip and said other side of said strip and engaging said plurality of metallic overlays on said one side of said strip; and g. a plurality of solder joints each connecting one of said plurality of metallic overlays and said fusible wire of silver. 